In my last post, which made it to Hacker News, I wrote:

"HE can have some funny side effects. In a project a connection to a
development web server sometimes worked and sometimes didn't. The
solution was quite simple. The customer used a split VPN tunnel.  IPv4 was
routed via the VPN tunnel and those IPv4 addresses were allowed in the web
servers access list. IPv6 was routed via the normal Internet connection and
those addresses weren't allowed."

This lead to some questions. Why didn’t HE work? Well it did work. Sometimes the IPv4 connection was better than the IPv6 connection, and sometimes IPv6 was better than IPv4. The TCP connection worked. And that is what counts for Happy Eyeballs.

Yesterday I read this toot (German) over on mastodon which starts with “IPv6 is hard.”

No it’s not. It’s different.

I ran across this multiple times: There is an A and an AAAA-record for a FQDN, but the web server is only reachable via IPv4. You can easily test this with curl

$ curl -4  https://github.com -o /dev/null
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  273k    0  273k    0     0  3417k      0 --:--:-- --:--:-- --:--:-- 3553k
$ curl -6  https://github.com -o /dev/null
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
curl: (7) Couldn't connect to server

When using IPv4 273k are “saved” to /dev/null, using IPv6 we get an error message “Couldn’t connect to server”

• CLAT in Linux Networkmanager
• DNS delegation
• jc - Convert command output to json
• Stratoshark - Analyse syscalls
• curl from start to end
• Security Bingo
• Linux Routing Fundamentals
• Packet where are you
• nftable-tracer

• https://github.com/ipinfo/cli
• [https://blog.sdn.clinic/2025/03/wireguard-and-vrfs/9(https://blog.sdn.clinic/2025/03/wireguard-and-vrfs/]
• https://github.com/girishji/pythondoc.vim

Some time ago I asked on Mastodon if someone would be interested in a series of videos about IPv6. Some people are, so I’ll give it a try. Each video will be accompanied by a blog post. This blog post and other materials will be available on gitlab.

Starting in January 2025, videos will come out every Tuesday evening (CET) and I’ll try to keep them between 10 and 20 minutes.